1.4ORIGIN OF MODERN

ASTRONOMY

The Roots of Astronomy• The study of the astronomy of ancient peoples has

been termed archeoastronomy.

▫Back in the stone and bronze ages, humans were interested in the cyclic motions in the sky.

▫Monuments dating back to ~3000 B.C. show alignments with astronomical significance.

Used for calendars or to predict eclipses.

Newgrange, Ireland built around 3200 B.C.

Sunlight shines down a central passageway on the day of the winter solstice.

The Roots of Astronomy• Perhaps the best known

example Stonehenge, located on Salisbury Plain in southern, England.

▫Built in stages from 3000 B.C. to 1800 B.C.

• Alignments with locations of sunset, sunrise, moonset, and moonrise at summer and winter solstices. Heelston

e

Summer Solstice

http://upload.wikimedia.org/wikipedia/commons/a/ae/Sun_behind_the_Heel_Stone.jpg

Other Examples• Chaco Canyon, in New

Mexico, features slits in the rock formation producing what is known as a “Sun Dagger”, indicating the day of the summer solstice.

Astronomy of Greece• The Golden Age of

astronomy originated and was centered in Greece.

▫First preserved written documents about ancient astronomy were from ancient Greek philosophy. Greeks tried to understand

the motions of the sky and describe them in terms of mathematical models, rather than physical models.

Greek Astronomers• Thales of Miletus (624-547 B.C.)

▫Universe is rational

▫Mysteries because they were unknown, not unknowable.

• Pythagoras (570-500 B.C.)

▫Geometric and mathematical relations Pythagorean Theorem

• Plato (428-347 B.C.)

▫Philosopher, not astronomer

▫ Influenced the notion of rotating spheres around Earth.

Greek Astronomers• Eudoxus (409-356 B.C.)

▫ Student of Plato; applied his principle of spheres and devised a model of 27 to show the motions of the universe around Earth.

• Aristotle (384-322 B.C.)

▫ Like most of his predecessors, Aristotle believed in a geocentric universe, one in which the Earth is at the center.

▫ Expanded the sphere model of Eudoxus to 55 total spheres, with fixed stars in the background.

▫ Knew Earth was spherical in shape due to the shadow it cast on the moon during lunar eclipses.

The Ancient Universe•Ancient astronomers believed Earth did not

move because they failed to study parallax, the apparent motion of an object due to the motion of the observer.

• Parallax is difficult to see with the unaided eye, thus the use of modern telescopes have helped us better determine distances to stars.

Greek Astronomers•Aristarchus (310-230 B.C.)

▫First to propose the Earth rotated on its axis and revolved around the Sun.

•Eratosthenes (276-194 B.C.)

▫Discovered a way to measure Earth’s radius.

▫Used the well at Syene (Aswan) in southern Egypt.

The Well At Syene• Eratosthenes learned

sunlight shone vertically into the well on the day of the summer solstice.

▫ Concluded the Sun was at the zenith at Syene.

▫ On that same day in Alexandria, he noted the Sun was about 7.2° south of the zenith.

• 360° (full circle)/7.2° = 50▫ Discovered the distance

from Alexandria to Syene was 1/50 of Earth’s circumference.

The Well At Syene• Travelers told him it took 50

days to travel from Alexandria to Syene; camel can travel 100 stadia (singular of stadium) per day

▫ 50 x 100 = 5000 stadia.

▫ If 5000 stadia = 1/50 of Earth’s circumference, then the total circumference = 250,000 stadia

• Circumference formula of a sphere = 2πr

▫ Found Earth’s radius = 40,000 stadia.

How Close Was Eratosthenes?• The stadium had different

lengths in ancient times, but if you assume 6 stadia to the kilometer, he was only off by 4%.

• If he used the Olympic stadium, his result was 14% too big.

• In any case, this was a much better measurement of Earth’s radius than Aristotle’s, which was much too small (40% of the true radius).

Later Refinements• Hipparchus (190-120 B.C.)

▫Credited with inventing trigonometry and creating the first star catalog (850 stars).

• Ptolemy (A.D. 90-168)

▫Created a mathematical model of the universe in which planets followed a small circle called the epicycle. This slid around a larger circle called the deferent.

Retrograde Motion• A big problem for ancient astronomers was planetary motion.

• Planets did not move at a constant rate; occasionally stop and move westward before resuming their eastward motion.

• This backward motion is called retrograde motion.

The Copernican Revolution• Nicholas Copernicus (1473-1543)

▫ Proposed a heliocentric universe, one in which the Sun was at the center.

▫ Explained daily and annual cycles of the sky by saying the Earth rotates on its axis and revolves around the Sun.

The Copernican Revolution•Copernicus could

explain retrograde motion without Ptolemy’s epicycles.

▫Occurs when Earth passes a planet.

▫However, Copernicus still was a firm believer of uniform circular motion.

Planetary Motion• The Copernican hypothesis solved

the problem of the place of Earth, but didn’t explain planetary motion.

▫ If the planets don’t move in uniform circular motion, how do they move?

• Tycho Brahe (1546-1601)

▫ Known for precise observations of the stars and planets.

▫ Built an observatory on the island of Hveen, just off the Danish coast. Became international center of

astronomical study.

Brahe’s Legacy• Still believed in the geocentric universe.

▫ Rejected the Ptolemaic universe; devised a model in which the Earth was the immobile center around which the Sun and moon moved. Other planets circled the Sun.

An Astronomer of Humble Origins• Brahe hired an assistant to prove

the validity of his hypothesis.

• Johannes Kepler (1571-1630)

▫ Became imperial mathematician when Brahe died in November of 1601, at his request.

▫ In college, he became a believer of the Copernican hypothesis (heliocentric). Abandoned both uniform motion and

circular motion.

Planets move around the Sun in elliptical paths with non-uniform velocities.

Kepler’s 3 Laws of Planetary Motion

1. The orbits of the planets are ellipses with the Sun at one focus.

The semimajor axis (a), is half of the longest diameter.

The eccentricity (e) of an ellipse is half the distance between the foci (c) divided by the semimajor axis (a).

c

Eccentricity = c/a

Eccentricities of Ellipses1)

e = 0.02

2)

e = 0.1

3)

e = 0.2

4)

e = 0.4

5)

e = 0.6

Eccentricities of Planetary Orbits•Orbits of planets are virtually

indistinguishable from circles.

Earth: e = 0.0167 Pluto: e = 0.248

Kepler’s 3 Laws of Planetary Motion

2. A line from a planet to the Sun sweeps out equal areas in equal intervals of time.

Kepler’s 3 Laws of Planetary Motion

3. A planet’s orbital period (P) squared is proportional to its semi-major axis (a), or average distance from the Sun, cubed.

P2 = a3

Py = period in years aAU = distance in AU

Telescopic Observations• Galileo Galilei (1564-1642)

▫Built his own telescope in 1609.

▫Observed astronomical features and reported major discoveries.

Discoveries of Galileo (1)• The moon was imperfect.

▫Mountains and valleys dominant.

▫Noticed other surface features on the moon as well.

Discoveries of Galileo (2)•4 major moons of Jupiter: Galilean moons

▫Io, Europa, Ganymede, Callisto

Rings of Saturn – What he saw

Galilean Moons of Jupiter

Discoveries of Galileo (3)•Found the Sun was also

imperfect.

▫Recognized dark regions on the Sun, now known as sunspots (cooler regions).

Discoveries of Galileo (4)• Recognized Venus goes through phases.

▫Similar to the moon signified Venus revolved around the Sun rather than the Earth.

Modern Astronomy• Began during the 99 years between the deaths of

Copernicus and Galileo (1543-1642).

• Marked the change from the Ptolemaic model of the universe to the Copernican model.

• This period marks the beginning of the modern scientific method.

• Scientists beginning with Copernicus and including Brahe, Kepler, and Galileo depended more on evidence, observation, and measurement rather than on first principles.